This invention relates to a method of protecting the catalytic activity of a SAPO molecular sieve, and to a method of making an olefin product by contacting the activated catalyst with an oxygenate feedstock. In particular, this invention relates to a method of protecting the catalytic activity of a SAPO molecular sieve by shielding the catalytic sites of the molecular sieve.
Silicoaluminophosphates (SAPOs) have been used as adsorbents and catalysts. As catalysts, SAPOs have been used in processes such as fluid catalytic cracking, hydrocracking, isomerization, oligomerization, the conversion of alcohols or ethers, and the alkylation of aromatics. In particular, the use of SAPOs in converting alcohols or ethers to olefin products, particularly ethylene and propylene, is becoming of greater interest for large scale, commercial production facilities.
As is known in the development of new large scale, commercial production facilities in the commodity chemical business, many problems arise in the scale up from laboratory and pilot plant operations. This is particularly a concern in catalytic reaction systems where large scale operation will be several orders of magnitude larger than typical pilot scale facilities. For example, conventional laboratory scale processes of making olefin products from oxygenate feed are conducted with catalyst loads of about 5 grams. Conventional large pilot plant operations may utilize as much as 50 kg of catalyst, making on the order of 20 kg/hr ethylene and propylene product, but this is nevertheless minuscule in comparison to what a large scale, commercial production facility would produce, if one were in existence today. It would be desirable to operate large scale, commercial production facilities, which may require a catalyst loading of anywhere from 1,000 kg to 700,000 kg, producing anywhere from 600 to 400,000 kg/hr of ethylene and propylene product, if a reliable method of providing such a large quantity of catalyst could be used.
Operating large scale, commercial production facilities clearly presents great challenges in the development of the catalyst production-to-use chain. By production-to-use chain is meant the entire area of activities beginning with the production of molecular sieve, including such activities as receipt of starting materials, on through the crystallization process. Also included in the production-to-use chain are intermediate activities which include formulation of the sieve with binders and other materials, activation of the manufactured sieve and finished catalyst; storage, transport, loading, unloading of molecular sieve and finished catalyst; as well as other practices associated with the handling and preparation of the sieve and finished catalyst for its ultimate use. The production-to-use chain ends at the point when the molecular sieve is introduced into the reaction system. For purposes of this invention, the end of the production-to-use chain does not necessarily mean the instant when the molecular sieve is introduced into the reaction system, since large scale systems are very large and instantaneous measurements are not practically feasible. In large scale systems, the production-to-use chain may be considered as completed some time within 12 hours of loading an activated catalyst into the reaction system.
Since information to date relating to production of olefin products by catalytic conversion of oxygenate feedstock has been limited to laboratory and small pilot plant activities, little if any attention has been paid to the problems associated with the intermediate activities in the production-to-use chain. For example, little attention has been focused on the impact of storage, transport, etc. on catalyst activity, since small scale activity is rather easily manageable. While today only relatively small quantities of catalyst are stored and transported, large quantities of materials will need to be handled for commercial operations. This may require storage of large quantities of sieve and catalyst materials for considerable periods of time, at multiple locations, and under rather rigorous industrial conditions.
As the management of sieve and catalyst in the catalyst production-to-use chain expands in volume and complexity, there is the likelihood that millions of dollars will be tied up in catalyst inventory, and the value of the sieve and catalyst will be lost if quality is not maintained at every step. Loss of quality will necessarily translate to loss of product quality as well as loss of product quantity, and these product losses could far outweigh the cost of the sieve and catalyst.
Although there has been some work published relating to the intermediate activities in the catalyst production-to-use chain, few of the problems associated therewith have been addressed. For example, U.S. Pat. No. 4,681,864 to Edwards et al. discuss the use of SAPO-37 molecular sieve as a commercial cracking catalyst. It is disclosed that activated SAPO-37 molecular sieve has poor stability, and that stability can be improved by using a particular activation process. In this process, organic template is removed from the core structure of the sieve just prior to contacting with feed to be cracked. The process calls for subjecting the sieve to a temperature of 400-800xc2x0 C. within the catalytic cracking unit.
U.S. Pat. No. 5,185,310 to Degnan et al. discloses another method of activating silicoaluminophosphate molecular sieve compositions. The method calls for contacting a crystalline silicoaluminophosphate with gel alumina and water, and thereafter heating the mixture to at least 425xc2x0 C. The heating process is first carried out in the presence of an oxygen depleted gas, and then in the presence of an oxidizing gas. The object of the heating process is to enhance the acid activity of the catalyst. The acid activity is enhanced as a result of the intimate contact between the alumina and the sieve.
Briend et al., J. Phys. Chem. 1995, 99, 8270-8276, teach that SAPO-34 loses its crystallinity when the template has been removed from the sieve and the de-templated, activated sieve has been exposed to air. Data are presented, however, which suggest that over at least the short term, this crystallinity loss is reversible. Even over a period of perhaps two years, the data suggest that crystallinity loss is reversible when certain templates are used.
EP-A2-0 203 005 also discusses the use of SAPO-37 molecular sieve in a zeolite catalyst composite as a commercial cracking catalyst. According to the document, if the organic template is retained in the SAPO-37 molecular sieve until a catalyst composite containing zeolite and the SAPO-37 molecular sieve is activated during use, and if thereafter the catalyst is maintained under conditions wherein exposure to moisture is minimized, the crystalline structure of the SAPO-37 zeolite composite remains stable.
As seen from the disclosure herein, we have now found that an activated SAPO molecular sieve will exhibit a loss of catalytic activity when exposed to a moisture-containing environment, and that this loss occurs between the time the catalyst is activated and even after as little as one day of storage. More importantly, we have now found that the loss of catalytic activity is not reversible after a certain period of time. It is desirable, therefore, to obtain an activated SAPO molecular sieve and incorporate that molecular sieve into a catalytic process before loss of catalytic activity becomes too great.
In order to overcome at least one of the many problems inherent in the prior art, the invention provides a method of protecting catalytic activity of a silicoaluminophosphate molecular sieve which is to be used in converting an oxygenate feedstock to an olefin product, particularly an olefin product comprising ethylene, propylene, or both. Protection against loss of catalytic activity is provided by covering catalytic sites of the molecular sieve with a shield prior to contacting with the oxygenate feedstock. Catalytic contact, i.e., contact of feedstock with molecular sieve under catalytic conversion conditions, must be made before a parameter defined herein as the methanol uptake index drops too low. In addition, the weight percent methanol conversion of the catalyst, determined at standard parameters, should not be allowed to drop below a minimum percentage. Drops in methanol uptake index or methanol conversion which are too low will likely result in a catalyst that is of little or no practical use in a large scale process.
In this invention, the shield can be provided in several ways. The shield can be the template material which is actually used to make the molecular sieve. As is known in the art, the template forms the porous structure within the molecular sieve. Conventionally, the template is removed by calcining, essentially burning it from the molecular sieve. Leaving the template within the intracrystalline structure for the proper time will, however, protect the catalytic sites that are within the porous structure of the molecular sieve.
Even if the template is removed the molecular sieve can still be protected by providing other types of shields to cover the catalytic sites. For example, carbonaceous material can be used as a shield. One way of providing the carbonaceous material is to partially calcine or burn the template, leaving enough carbon material within the pores of the molecular sieve to provide the shield.
An anhydrous environment can also act as a shield, even when the template or carbonaceous material has been removed. An anhydrous environment is one that is depleted in water content. It can be either a gas or a liquid environment.
In a particular embodiment of protecting catalytic activity of a silicoaluminophosphate molecular sieve, the invention comprises providing a silicoaluminophosphate molecular sieve having catalytic sites protected against loss of catalytic activity by covering with a shield, and introducing the molecular sieve into an oxygenate reaction system, wherein the molecular sieve has a methanol uptake index of at least 0.15 at time of contact with oxygenate under conditions effective to convert the oxygenate to olefin product. To provide protection for storage and transportation the shield should be provided within the molecular sieve for at least 12 hours prior to contact with oxygenate. Longer storage and transportation conditions may require that the shield be provided within the shield for longer periods of time, e.g., 24 hours, 1 month, or perhaps many months.
The protected molecular sieve is of great benefit in large scale commercial processes of making olefin product from oxygenate feedstock, particularly making olefins containing ethylene or propylene from feedstock comprising methanol or dimethyl ether. In a particular embodiment of making an olefin product from an oxygenate feedstock, the invention is to a method which comprises providing a silicoaluminophosphate molecular sieve having catalytic sites within the molecular sieve; providing a shield to protect the catalytic sites from contact with water molecules; removing the shield; and, after removing the shield, contacting the sieve with an oxygenate feedstock under conditions effective to produce an olefin product, wherein the activated sieve contacting the oxygenate feedstock has a methanol uptake index of at least 0.15, preferably 0.4, more preferably at least 0.6, and most preferably at least 0.8.
In another embodiment, there is provided a method of making an olefin product from an oxygenate feedstock, comprising removing a template from a silicoaluminophosphate molecular sieve and contacting the molecular sieve with the oxygenate feedstock under conditions effective to convert the feedstock to an olefin product before the methanol uptake index drops below 0.15, preferably 0.4, more preferably 0.6, most preferably 0.8.
It is desirable that the activated molecular sieve that is contacted with oxygenate feedstock have a methanol conversion of at least 10 wt. % at a standard time on stream (TOS) of 5 minutes and a weight hourly space velocity (WHSV) of 25 hrxe2x88x921. Preferably the molecular sieve should have a methanol conversion of at least 15 wt. % at a standard time on stream of 5 minutes and a WHSV of 25 hrxe2x88x921, more preferably a methanol conversion of at least 20 wt. % at a standard time on stream of 5 minutes and a WHSV of 25 hrxe2x88x921.
The shield can be removed ex situ (outside the reactor per se) or in situ. In a preferred embodiment, the shield is the template and the template is removed outside of the reactor unit per se in order to minimize product contamination, particularly nitrogen contamination due to nitrogen components within a nitrogen-containing template that may be used as the shield.
In another preferred embodiment, once the shield has been removed, the molecular sieve can be maintained at a temperature of at least 150xc2x0 C., with no shield, with little if any catalyst activity loss due to exposure of catalytic sites with moisture. In this embodiment, the molecular sieve is preferably maintained at a temperature of 150 to 800xc2x0 C., more preferably at a temperature of 175-600xc2x0 C., and most preferably at a temperature of 200-500xc2x0 C. in order to maintain catalyst activity.
Preferably, the template is a nitrogen-containing hydrocarbon. Preferably, the nitrogen-containing hydrocarbon is selected from the group consisting of a tetraethyl ammonium hydroxide salt, cyclopentylamine, aminomethyl cyclohexane, piperidine, triethylamine, cyclohexylamine, tri-ethyl hydroxyethylamine, morpholine, dipropylamine, pyridine, isopropylamine and mixtures thereof. The silicoaluminophosphate molecular sieve is preferably selected from the group consisting of SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, metal containing forms thereof, and mixtures thereof.
The oxygenate feedstock is preferably selected from the group consisting of methanol; ethanol; n-propanol; isopropanol; C4-C20 alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic acid; and mixtures thereof More preferably, the oxygenate feedstock is methanol or dimethyl ether.
The silicoaluminophosphate molecular sieve can provided with a binder material, and the template can be removed by heating at a temperature between 200xc2x0 C. and 800xc2x0 C. In order to convert the oxygenate to olefin product, the process is preferably performed at a temperature between 200xc2x0 C. and 700xc2x0 C.
The present invention will be better understood by reference to the Detailed Description of the Invention when taken together with the attached drawings and the appended claims.